Electromagnetic (EM) measurement systems for geophysical measurement purposes detect the electric and magnetic fields that can be measured in, on or above the earth, to identify subsurface changes in electrical properties of materials beneath the earth's surface. Airborne EM systems carry out field measurements in the air above the earth. A primary goal is to make measurements at a number of spatial locations to identify the size and position of localized material property changes. Such changes can be attributed to a desired outcome such as identifying a localized mineral deposit, a buried object, or the presence or absence of water.
Generally speaking, EM systems usually include a source of electromagnetic energy (transmitter) and a receiver to detect the response of the ground. EM systems can be either frequency-domain or time-domain. Both types of systems are based on principles encapsulated in Faraday's Law of electromagnetic induction, which states that a time-varying primary magnetic field will produce an electric field. For airborne systems, the primary field is created by passing a current through a transmitter loop (or series of transmitter loops). The temporal changes to the created or radiated magnetic field induce electrical eddy currents in the ground. These currents have an associated secondary magnetic field that can be sensed, together with the primary field, by a series of receiver coils.
Each receiver coil may consist of a series of wire loops, in which a voltage is induced proportional to the strength of the eddy currents in the ground and their rate of change with time. Typical receiver coils have axes in the three Cartesian directions that are orthogonal to one another. Coils with their axes perpendicular to the earth are most sensitive to horizontal layers and half-spaces. Coils with their axes horizontal are more sensitive to discrete or vertical conductors.
In frequency-domain systems, the time-varying transmitter signal is a sinusoidal waveform of constant frequency, inducing electrical currents in the ground of the same frequency. Most systems use several constant frequencies that are treated independently. Although the secondary field has the same frequency as the primary field, it will have a different amplitude and phase.
For time-domain systems, a time-varying field is created by a current that may be pulsed. The change in the transmitted current induces an electrical current in the ground that persists after the primary field is turned off. Typical time domain receiver coils measure the rate of change of this secondary field. The time-domain transmitter current waveform repeats itself periodically and can be transformed to the frequency domain where each harmonic has a specific amplitude and phase.
Existing prior art EM systems have limitations in surveying various terrains and geologies. For example, time domain EM systems existing in the prior art are typically configured or optimized to measure a particular type of terrain or geology near an estimated depth, based on a number of considerations pertinent to each task. These time domain EM systems generally are not well-equipped to deal with surveys of complex geology, which may comprise a mixture of deep and shallow geological structures, and/or strong and weak conductivities.
As a result, in some surveys of complex terrain or geology where existing prior art time domain EM systems were used, the geology of interest would be flown over multiple times, each with a specifically configured EM system for detecting one specific aspect of the terrain or geology. While this approach may provide desirable survey resolution, it is generally time consuming and not cost-effective.
In some ground-based EM systems existing in the prior art, such as the Geonics™ EM-37 system, two or more transmitter waveforms were used to collect shallow and deep ground information, wherein repeated pulses of a first waveform are transmitted and measured, followed by the transmission and measurement of repeated pulses of a second waveform. In contrast, for an airborne EM system which is constantly moving, it may be difficult to use a dual waveform system to collect responses from both waveforms over the same geology, which results in poor survey resolution for the combined waveform survey data.
Therefore, there remains a need for an improved EM surveying system that can efficiently and cost-effectively provide measurements of various terrains and complex geologies.